A United Nations rapporteur says tough antidrug laws are likely behind a "staggering surge" in executions in recent years.

Ahmed Shaheed, in a report released on March 10, said 966 people were executed in Iran last year, up from 697 in 2013 and 91 in 2005.

The "staggering surge" in executions, including more than half of last year's total, were due to drug violations, he said.

"Under Iran's current drug laws, possession of 30 grams of heroine or cocaine would qualify for the death penalty. So there's a number of draconian laws," he said.

But he noted that hundreds of journalists, bloggers, activists, and opposition figures also "currently languish in Iran's prisons," some under the threat of execution.

Shaheed said executions of juvenile offenders were "strictly and unequivocally prohibited" under international law. He said at least 73 had been executed in Iran between 2005 and 2015, the highest number of any country.

Shaheed has not been allowed to visit Iran as part of his mandate, now in its fifth year.